Optical recording medium

ABSTRACT

An optical recording medium includes a substrate, a light transmission layer and a plurality of recording layers between the substrate and the light transmission layer and capable of recording data in the plurality of recording layers and reproducing data recorded in the plurality of recording layers by projecting a laser beam via the light transmission layer onto the plurality of recording layers, at least one recording layer other than a farthest recording layer from the light transmission layer including a reflective film containing Ag as a primary component and C as an additive. In the thus constituted optical recording medium, recording characteristics and reproducing characteristic of the respective recording layers can be improved.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an optical recording medium and,particularly, to an optical recording medium having a plurality ofrecording layers and in which recording characteristics and reproducingcharacteristic of the respective recording layers can be improved.

DESCRIPTION OF THE PRIOR ART

[0002] Optical recording media such as the CD, DVD and the like havebeen widely used as recording media for recording digital data. Suchoptical recording media require improvement in ability to record largeamounts of data and various proposals have been made in order toincrease the data recording capacity thereof

[0003] One of these is an optical recording medium having two recordinglayers and such an optical recording medium has been already put to thepractical use as an optical recording medium adapted to enable only datareading, such as the DVD-Video and the DVD-ROM.

[0004] An optical recording medium adapted only for reading data andprovided with two recording layers is formed by laminating twosubstrates each having prepits constituting a recording layer on thesurface thereof via an intermediate layer.

[0005] Further, an optical recording medium having two recording layershas been recently proposed in connection with optical recording media inwhich data can be rewritten by the user (See Japanese Patent ApplicationLaid Open No. 2001-243655 etc.).

[0006] A rewritable type optical recording medium having two recordinglayers is constituted by laminating recording layers each including arecording film sandwiched between dielectric layers (protective layers)via an intermediate layer.

[0007] In the case where data are to be recorded in a rewritable typeoptical recording medium having two recording layers, the phase state ofone of the recording layers is changed by adjusting the focus of a laserbeam onto the one of the recording layers, setting the power of thelaser beam to a recording power Pw whose level is sufficiently higherthan a reproducing power Pr and, projecting the laser beam onto therecording layer to form a record mark at a predetermined region of therecording layer.

[0008] Since the reflection coefficient of the thus formed record markis different from those of blank regions in which no record mark isformed, it is possible to reproduce data recorded in one of therecording layers by adjusting the focus of the laser beam onto therecording layer, projecting the laser beam whose power is set to thereproducing power Pr and detecting the amount of light reflected by therecording layer.

[0009] In this manner, in the rewritable type optical recording mediumhaving two recording layers, since data are recorded in one of therecording layers and data recorded in the recording layer are reproducedby adjusting the focus of the laser beam onto the recording layer andprojecting the laser beam onto the recording layer, when data are to berecorded in the farther recording layer (hereinafter referred to as the“L1 layer”) from the light incident plane and data are producedtherefrom, the laser beam is projected via the closer recording layer(hereinafter referred to as the “L0 layer”) to the light incident planeonto the L1 layer.

[0010] Therefore, in order to record data in the L1 layer and reproducedata from the L1 layer in a desired manner, it is necessary for the L0layer to have a sufficiently high light transmittance for the laser beamand the L0 layer is required to have a thin reflective film.

[0011] However, as the reflective film included in the L0 layer is madethinner, heat generated in the recording film included in the L0 layerby the laser beam is not readily radiated. On the other hand, as thereflective film included in the L0 layer is made thicker, the lighttransmittance of the L0 layer is inevitably lowered.

[0012] In the case where the L0 layer does not have a sufficiently heatradiation characteristic, since heat generated by the irradiation withthe laser beam is stored excessively in the recording film included inthe L0 layer, the width of a record mark recorded in the recording filmbecomes larger and, therefore, data cannot be recorded therein in adesired manner. In the case where the heat radiation characteristic ofthe L0 layer is particularly low, the recording film once changed to anamorphous state is again crystallized in a recording film containing aphase change material and it is impossible to form a record mark havinga desired shape.

[0013] In view of the above, it has been difficult to simultaneouslyimprove the recording characteristic and the reproducing characteristicof the L0 layer and those of the L1 layer.

[0014] The same problems also occur in a write-once type opticalrecording medium having two or more recording layers and an opticalrecording medium having an L1 layer for reading data only and it hasbeen desired to solve there problems.

SUMMARY OF THE INVENTION

[0015] It is therefore an object of the present invention to provide anoptical recording medium having a plurality of recording layers and inwhich recording characteristics and reproducing characteristic of therespective recording layers can be improved.

[0016] The above and other objects of the present invention can beaccomplished by an optical recording medium comprising a substrate, alight transmission layer and a plurality of recording layers between thesubstrate and the light transmission layer and capable of recording datain the plurality of recording layers and reproducing data recorded inthe plurality of recording layers by projecting a laser beam via thelight transmission layer onto the plurality of recording layers, atleast one recording layer other than a farthest recording layer from thelight transmission layer including a reflective film containing Ag as aprimary component and C as an additive.

[0017] It is necessary to form a reflective film of a material having ahigh light transmittance and a high thermal conductivity in order tosimultaneously improve the recording characteristic and the reproducingcharacteristic of the L0 layer and those of the L1 layer and Ag is atypical material having such characteristics. However, Ag corrodeseasily. Therefore, when the reflective film included in the L0 layer isformed of Ag, the storage reliability of the optical recording medium islow. However, according to the present invention, since at least onerecording layer other than a farthest recording layer from the lighttransmission layer includes a reflective film containing Ag as a primarycomponent and C as an additive, it is possible to improve the lighttransmittance and the thermal conductivity of the reflective filmincluded in the L0 layer while preventing the reflective film from beingcorroded and it is therefore possible to improve recordingcharacteristics and reproducing characteristic of the respectiverecording layers and improve the storage reliability of the opticalrecording medium.

[0018] In particular, since energy per unit area of the spot of a laserbeam is high in a next-generation type optical recording medium in whichthe thickness of the light transmission layer is set at 30 μm to 200 μmand data are recorded therein and reproduced therefrom by condensing alaser beam having a wavelength equal to or shorter than 450 nm using anobjective lens having a numerical aperture equal to or larger than 0.7,it is required for the next-generation type optical recording medium tohave a high heat radiation characteristic and, therefore, the presentinvention can be particularly effectively applied to the next-generationtype optical recording medium.

[0019] Further, according to the present invention, since the reflectivefilm contains Ag as a primary component and inexpensive C as anadditive, it is possible to prevent material cost of the opticalrecording medium from increasing.

[0020] In the present invention, it is preferable that the reflectivefilm included in the at least one recording layer other than thefarthest recording layer from the light transmission layer contains 0.5atomic % to 5.0 atomic % of C. When the amount of C added to thereflective film exceeds 5.0 atomic %, it is difficult to form areflective film having sufficiently high light transmittance and thermalconductivity and, on the other hand, when the amount of C added to thereflective film is less than 0.5 atomic %, the storage reliability ofthe optical recording medium becomes low.

[0021] In the present invention, it is more preferable that thereflective film contains 1.0 atomic % to 4.0 atomic % of C and it isparticularly preferable that the reflective film contains about 2.5atomic % of C. When the amount of C added to the reflective film isequal to or less than 4.0 atomic %, it is possible to form a reflectivefilm having light transmittance close to that of a reflective filmformed of pure Ag and on the other hand, when the amount of C added tothe reflective film exceeds 2.0 atomic %, improvement in the storagereliability of the optical recording medium is small even if the amountof C is increased. When the reflective film contains about 2.5 atomic %of C, it is possible to form a reflective film having substantially thesame light transmittance as that of a reflective film formed of pure Agand obtain the optical recording medium having high storage reliability.

[0022] In a preferred aspect of the present invention, each of theplurality of recording layers contains a phase change material.

[0023] In a further preferred aspect of the present invention, each ofthe plurality of recording layers contains a SbTe system material.

[0024] In a further preferred aspect of the present invention, each ofthe plurality of recording layers contains a SbTe system materialrepresented by a compositional formula: (Sb_(x)Te_(1−x))_(1−y)M_(y)wherein M is an element other than Sb and Te, x is equal to or largerthan 0.55 and equal to or smaller than 0.9 and y is equal to or largerthan 0 and equal to or smaller than 0.25.

[0025] In a further preferred aspect of the present invention, M isconstituted as one or more elements selected from the group consistingof In, Ag, Au, Bi, Se, Al, P, Ge, H, Si, C, V, W, Ta, Zn, Mn, Ti, Sn,Pd, N, O and rare earth elements.

[0026] In another preferred aspect of the present invention, each of theplurality of recording layers includes a first recording film containingone kind of element selected from the group consisting of Si, Ge, Sn,Mg, C, Al, Zn, In, Cu and Bi as a primary component and a secondrecording film disposed in the vicinity of the first recording film andcontaining one kind of element from the group consisting of Cu, Al, Zn,Si and Ag and different from the element contained in the firstrecording film as a primary component and the element contained in thefirst recording film as a primary component and the element contained inthe second recording film as a primary component are mixed when thefirst recording film and the second recording film is irradiated with alaser beam, thereby forming a record mark.

[0027] In a further preferred aspect of the present invention, the firstrecording film contains an element selected from the group consisting ofSi, Ge, Sn, Mg, In, Zn, Bi and Al as a primary component and the secondrecording film contains Cu as a primary component.

[0028] In a further preferred aspect of the present invention, the firstrecording film contains an element selected from the group consisting ofSi, Ge, Sn, Mg and Al as a primary component.

[0029] In a further preferred aspect of the present invention, anelement selected from the group consisting of Al, Si, Zn, Mg, Au, Sn,Ge, Ag, P, Cr, Fe and Ti is added to the second recording filmcontaining Cu as a primary component.

[0030] In a further preferred aspect of the present invention, anelement selected from the group consisting of Al, Zn, Sn and Au is addedto the second recording film containing Cu as a primary component.

[0031] In another preferred aspect of the present invention, the firstrecording film contains an element selected from the group consisting ofSi, Ge, C, Sn, Zn and Cu as a primary component and the second recordingfilm contains Al as a primary component.

[0032] In a further preferred aspect of the present invention, anelement selected from the group consisting of Mg, Au, Ti and Cu is addedto the second recording film containing Al as a primary component.

[0033] In another preferred aspect of the present invention, the firstrecording film contains an element selected from the group consisting ofSi, Ge, C and Al as a primary component and the second recording filmcontains Zn as a primary component.

[0034] In a further preferred aspect of the present invention, anelement selected from the group consisting of Mg, Cu and Al is added tothe second recording film containing Zn as a primary component.

[0035] In a further preferred aspect of the present invention, the firstrecording film and the second recording film are formed so that a totalthickness thereof is 2 nm to 40 nm.

[0036] In the present invention, it is preferable that the lighttransmission layer has a thickness of 30 μm to 200 μm.

[0037] The above and other objects and features of the present inventionwill become apparent from the following description made with referenceto the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 is a schematic cross-sectional view showing the structureof an optical recording medium that is a preferred embodiment of thepresent invention.

[0039]FIG. 2 is a schematic enlarged cross-sectional view of the opticalrecording medium shown in FIG. 1.

[0040]FIG. 3 is a drawing showing a step of a method for fabricating anoptical recording medium which is a preferred embodiment of the presentinvention.

[0041]FIG. 4 is a drawing showing a step of a method for fabricating anoptical recording medium which is a preferred embodiment of the presentinvention.

[0042]FIG. 5 is a drawing showing a step of a method for fabricating anoptical recording medium which is a preferred embodiment of the presentinvention.

[0043]FIG. 6 is a drawing showing a step of a method for fabricating anoptical recording medium which is a preferred embodiment of the presentinvention.

[0044]FIG. 7 is a schematic cross-sectional view showing an opticalrecording medium which is another preferred embodiment of the presentinvention.

[0045]FIG. 8 is a schematic enlarged cross-sectional view of the opticalrecording medium shown in FIG. 7.

[0046]FIG. 9 is a graph showing how light transmittance of a reflectivefilm varies with amount of C added to the reflective film.

[0047]FIG. 10 is a graph showing how rate of reflection coefficientreduction of a reflective film varies with amount of C added to thereflective film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048]FIG. 1 is a schematic cross-sectional view showing the structureof an optical recording medium that is a preferred embodiment of thepresent invention and FIG. 2 is a schematic enlarged cross-sectionalview of the optical recording medium shown in FIG. 1.

[0049] As shown in FIGS. 1 and 2, an optical recording medium 10according to this embodiment is constituted as a rewritable type opticalrecording medium and includes a disk-like support substrate 11, atransparent intermediate layer 12, a light transmission layer 13, an L0layer 20 formed between the transparent layer 12 and the lighttransmission layer 13, and an L1 layer 30 formed between the supportsubstrate 11 and the transparent intermediate layer 12.

[0050] The L0 layer 20 and the L1 layer 30 are recording layers in whichdata are recorded, i.e., the optical recording medium 10 according tothis embodiment includes two recording layers.

[0051] The L0 layer 20 constitutes a recording layer close to a lightincident plane 13 a and as shown in FIGS. 1 and 2, the L0 layer 20 isconstituted by laminating a reflective film 21, a second dielectric film22, an L0 recording film 23 and a first dielectric film 24 from the sideof the support substrate 11.

[0052] On the other hand, the L1 layer 30 constitutes a recording layerfar from the light incident plane 13 a and as shown in FIGS. 1 and 2,the L1 layer 30 is constituted by laminating a reflective film 31, afourth dielectric film 32, an L1 recording film 33 and a thirddielectric film 34.

[0053] In the case where data are to be recorded in the L1 layer 30 anddata recorded in the L1 layer 30 are to be reproduced, a laser beam L isprojected thereon through the L0 layer 20 located closer to the lighttransmission layer 13.

[0054] Therefore, it is necessary for the L0 layer 20 to have a highlight transmittance. Concretely, the L0 layer 20 has a lighttransmittance equal to or higher than 30% with respect to the laser beamL used for recording data and reproducing data and preferably has alight transmittance equal to or higher than 40%.

[0055] In order to record data with high density, it is necessary toreduce the diameter of the laser beam L and, therefore, the laser beam Lused for recording data and reproducing data has a wavelength equal toor shorter than 500 nm and preferably a wavelength of 200 to 450 nm.

[0056] The support substrate 11 serves as a support for ensuringmechanical strength required for the optical recording medium 10.

[0057] The material used to form the support substrate 11 is notparticularly limited insofar as the support substrate 11 can serve asthe support of the optical recording medium 10. The support substrate 11can be formed of glass, ceramic, resin or the like. Among these, resinis preferably used for forming the support substrate 11 since resin canbe easily shaped. Illustrative examples of resins suitable for formingthe support substrate 11 include polycarbonate resin, acrylic resin,epoxy resin, polystyrene resin, polyethylene resin, polypropylene resin,silicone resin, fluoropolymers, acrylonitrile butadiene styrene resin,urethane resin and the like. Among these, polycarbonate resin is mostpreferably used for forming the support substrate 11 from the viewpointof easy processing, optical characteristics and the like and in thisembodiment, the support substrate 11 is formed of polycarbonate resin.In this embodiment, since the laser beam L is projected via the lightincident plane 13 a located opposite to the support substrate 11, it isunnecessary for the support substrate 11 to have a light transmittanceproperty.

[0058] In this embodiment, the support substrate 11 has a thickness ofabout 1.1 mm.

[0059] As shown in FIG. 1, grooves 11 a and lands 11 b are alternatelyformed on the surface of the support substrate 11. The grooves 11 aand/or lands 11 b serve as a guide track for the laser beam L when dataare to be recorded or when data are to be reproduced.

[0060] The depth of the groove 11 a is not particularly limited and ispreferably set to 10 nm to 40 nm. The pitch of the grooves 11 a is notparticularly limited and is preferably set to 0.2 μm to 0.4 μm.

[0061] The transparent intermediate layer 12 serves to space the L0layer 20 and the L1 layer 30 apart by a physically and opticallysufficient distance.

[0062] As shown in FIG. 1, grooves 12 a and lands 12 b are alternatelyformed on the surface of the transparent intermediate layer 12. Thegrooves 12 a and/or lands 12 b formed on the surface of the transparentintermediate layer 12 serve as a guide track for the laser beam L whendata are to be recorded or when data are to be reproduced.

[0063] The depth of the groove 12 a and the pitch of the grooves 12 acan be set to be substantially the same as those of the grooves 11 aformed on the surface of the support substrate 11.

[0064] It is preferable to form the transparent intermediate layer 12 soas to have a thickness of 10 μm to 50 μm and it is more preferable toform it so as to have a thickness of 15 μm to 40 μm.

[0065] The material for forming the transparent intermediate layer 12 isnot particularly limited and an ultraviolet ray curable acrylic resin ispreferably used for forming the transparent intermediate layer 12.

[0066] It is necessary for the transparent intermediate layer 12 to havesufficiently high light transmittance since the laser beam L passesthrough the transparent intermediate layer 12 when data are to berecorded in the L1 layer 30 and data recorded in the L1 layer 30 are tobe reproduced.

[0067] The light transmission layer 13 serves to transmit the laser beamL and the light incident plane 13 a is constituted by one of thesurfaces thereof.

[0068] It is preferable to form the light transmission layer 13 so as tohave a thickness of 30 μm to 200 μm.

[0069] The material for forming the light transmission layer 13 is notparticularly limited and, similarly to the transparent intermediatelayer 12, an ultraviolet ray curable acrylic resin is preferably usedfor forming the light transmission layer 13.

[0070] It is necessary for the light transmission layer 13 to havesufficiently high light transmittance since the laser beam L passesthrough the transparent intermediate layer 13 when data are to berecorded in the L1 layer 30 and data recorded in the L1 layer 30 are tobe reproduced.

[0071] Each of the L0 recording film 23 of the L0 layer 20 and the L1recording film 33 of the L1 layer 30 is formed of a phase changematerial. Utilizing the difference in the reflection coefficientsbetween the case where the L0 recording film 23 and the L1 recordingfilm 33 are in a crystal phase and the case where they are in anamorphous phase, data are recorded in the L0 recording film 23 and theL1 recording film 33 and data are reproduced from the L0 recording film23 and the L1 recording film 33.

[0072] The material for forming the L0 recording film 23 and the L1recording film 33 is not particularly limited but a material capable ofchanging from an amorphous phase to a crystal phase in a short time ispreferable in order to enable direct overwriting of data at a highvelocity. Illustrative examples of materials having such acharacteristic include a SbTe system material.

[0073] As the SbTe system material, SbTe may be used alone or a SbTesystem material to which additives are added in order to shorten timerequired for crystallization and improve the long-term storagereliability of the optical recording medium 10 may be used.

[0074] Concretely, it is preferable to form the L0 recording film 23 andthe L1 recording film 33 of a SbTe system material represented by thecompositional formula: (Sb_(x)Te_(1−x))_(1−y)M_(y), where M is anelement other than Sb and Te, x is equal to or larger than 0.55 andequal to or smaller than 0.9 and y is equal to or larger than 0 andequal to or smaller than 0.25, and it is more preferable to form the L0recording film 23 and the L1 recording film 33 of a SbTe system materialrepresented by the above mentioned compositional formula wherein x isequal to or larger than 0.65 and equal to or smaller than 0.85 and y isequal to or larger than 0 and equal to or smaller than 0.25.

[0075] While M is not particularly limited, it is preferable for theelement M to be one or more elements selected from the group consistingof In, Ag, Au, Bi, Se, Al, P, Ge, H, Si, C, V, W, Ta, Zn, Mn, Ti, Sn,Pd, N, O and rare earth elements in order to shorten time required forcrystallization and improve the storage reliability of the opticalrecording medium 10. It is particularly preferable for the element M tobe one or more elements selected from the group consisting of Ag, In, Geand rare earth elements for improving the storage reliability of theoptical recording medium 10.

[0076] Since the laser beam L passes through the L0 recording film 23when data are to be recorded in the L1 layer 30 and data recorded in theL1 layer 30 are to be reproduced, it is necessary for the L0 recordingfilm 23 to have a high light transmittance and it is thereforepreferable to form the L0 recording film 23 so as to be thinner than theL1 recording film 33.

[0077] Concretely, it is preferable to form the L1 recording film 33 soas to have a thickness of 3 to 20 nm and form the L0 recording film 23so as to have a thickness of 3 nm to 10 nm.

[0078] In the case where the thickness of the L0 recording film 23 andthe L1 recording film 33 is thinner than 3 nm, the L0 recording film 23and the L1 recording film 33 do not readily crystallize because a phasechange material does not crystallize easily if it is formed extremelythin. On the other hand, in the case where the thickness of the L0recording film 23 exceeds 10 nm, the light transmittance of the L0recording film 23 is lowered and the characteristic of recording data inthe L1 layer 30 and the characteristic of reproducing data from the L1layer 30 are degraded.

[0079] The first dielectric film 24 and the second dielectric film 22serve as protective layers for protecting the L0 recording film 23 andthe third dielectric film 34 and the fourth dielectric film 32 serve asprotective layers for protecting the L1 recording film 33.

[0080] The thickness of each of the first dielectric film 24, the seconddielectric film 22, the third dielectric film 34 and the fourthdielectric film 32 is not particularly limited and it preferably has athickness of 1 nm to 50 nm. In the case where the thickness of each ofthe first dielectric film 24, the second dielectric film 22, the thirddielectric film 34 and the fourth dielectric film 32 is thinner than 1nm, each of the first dielectric film 24, the second dielectric film 22,the third dielectric film 34 and the fourth dielectric film 32 does notsufficiently serve as a protective layer and is cracked during aninitialization process described later and the characteristic (repeatedoverwriting characteristic) of the optical recording medium 10 whendirect overwriting is repeated is degraded. On the other hand, in thecase where the thickness of each of the first dielectric film 24, thesecond dielectric film 22, the third dielectric film 34 and the fourthdielectric film 32 exceeds 50 nm, a long time is required for formingit, thereby lowering the productivity of the optical recording medium 10and there is some risk of cracking the L0 recording film 23 and the L1recording film 33 due to internal stress.

[0081] The first dielectric film 24, the second dielectric film 22, thethird dielectric film 34 and the fourth dielectric film 32 may have asingle-layered structure or may have a multi-layered structure includinga plurality of dielectric films. For example, if the first dielectricfilm 24 is constituted by two dielectric films formed of materialshaving different refractive indexes, light interference effect can beincreased.

[0082] The material for forming the first dielectric film 24, the seconddielectric film 22, the third dielectric film 34 and the fourthdielectric film 32 is not particularly limited but it is preferable toform the first dielectric film 24, the second dielectric film 22, thethird dielectric film 34 and the fourth dielectric film 32 of oxide,sulfide, nitride of Al, Si, Ce, Zn, Ta, Ti and the like such as Al₂O₃,AlN, SiO₂, Si₃N₄, CeO₂, ZnS, TaO and the like or a combination thereofand it is more preferable for them to contain ZnS.SiO₂ as a primarycomponent. ZnS.SiO₂ means a mixture of ZnS and SiO₂.

[0083] The reflective film 21 included in the L0 layer 20 serves toreflect the laser beam L entering the light incident plane 13 a so as toemit it from the light incident plane 13 a and effectively radiate heatgenerated in the L0 recording film 23 by the irradiation with the laserbeam L.

[0084] When data are to be recorded in the L1 layer 30 and data recordedin the L1 layer 30 are to be reproduced, the laser beam L entering thelight incident plane 13 a impinges onto the L1 layer 30 via thereflective film 21. It is therefore necessary to form the reflectivefilm 21 of a material having a high light transmittance and a highthermal conductivity. Further, it is necessary to form the reflectivefilm 21 of a material having long-term storage reliability.

[0085] Therefore, in this embodiment, the reflective film 21 is formedof a material containing Ag as a primary component and C as an additive.

[0086] In the case where the reflective film 21 is formed of a materialcontaining Ag as a primary component and C as an additive, it ispossible to markedly improve the long-term storage reliability of thereflective film 21 without degrading the inherent high lighttransmittance and high thermal conductivity of the Ag. Further, materialcost is not increased by adding C to the reflective film 21.

[0087] Therefore, if the reflective film 21 is formed of a materialcontaining Ag as a primary component and C as an additive, a reflectivefilm having a high light transmittance and a high thermal conductivitycan be formed and the storage reliability of the optical recordingmedium 10 can be improved.

[0088] In a study carried out by the inventors of the present invention,it was found that the light transmittance and the thermal conductivityof the reflective film 21 tended to decrease as the amount of C added tothe reflective film 21 increased and it was further found that when theamount of C added to the reflective film 21 was equal to or less than apredetermined value, the storage reliability of the optical recordingmedium 10 was improved as the amount of C added to the reflective film21 increased and when the amount of C added to the reflective film 21exceeded the predetermined value, improvement in the storage reliabilityof the optical recording medium 10 was not remarkable even if the amountof C added to the reflective film 21 was increased.

[0089] More specifically, when the amount of C added to the reflectivefilm 21 exceeds 5.0 atomic %, it is difficult to form the reflectivefilm 21 to have sufficiently high light transmittance and thermalconductivity and, on the other hand, when the amount of C added to thereflective film 21 is less than 0.2 atomic %, the storage reliability ofthe optical recording medium 10 becomes low. Therefore, it is preferablethat the reflective film 21 contains 0.5 atomic % to 5.0 atomic % of C.

[0090] Further, when the amount of C added to the reflective film 21 isequal to or less than 4.0 atomic %, it is possible to form thereflective film 21 to have a light transmittance close to that of areflective film 21 formed of pure Ag and, on the other hand, when theamount of C added to the reflective film 21 exceeds 2.0 atomic %,improvement in the storage reliability of the optical recording medium10 is small even if the amount of C is increased. Furthermore, when thereflective film 21 contains about 2.5 atomic % of C, it is possible toform the reflective film 21 having substantially the same lighttransmittance as that of a reflective film 21 formed of pure Ag andobtain an optical recording medium 10 having high storage reliability.Therefore, it is more preferable that the reflective film 21 contains1.0 atomic % to 4.0 atomic % of C and it is particularly preferable thatthe reflective film 21 contains about 2.5 atomic % of C.

[0091] Since the light transmittance and thermal conductivity of thereflective film 21 varies depending upon the amount of C added to thereflective film 21, the thickness of the reflective film 21 isdetermined based on the amount of C added to the reflective film 21 but,normally, the thickness of the reflective film 21 is preferably thinnerthan 20 nm and more preferably 5 nm to 15 nm.

[0092] The reflective film 31 included in the L1 layer 30 serves toreflect the laser beam L entering through the light incident plane 13 aso as to emit it from the light incident plane 13 a and effectivelyradiate heat generated in the L1 recording film 33 by the irradiationwith the laser beam L.

[0093] The reflective film 31 is preferably formed so as to have athickness of 20 nm to 200 nm. When the reflective film 31 is thinnerthan 20 nm, it does not readily radiate heat generated in the L1recording film 33. On the other hand, when the reflective film 31 isthicker than 200 nm, the productivity of the optical recording medium 10is lowered since a long time is required for forming the reflective film31 and there is a risk of cracking the reflective film 31 due tointernal stress or the like.

[0094] The material for forming the reflective film 31 is notparticularly limited. The reflective film 31 may be formed of the samematerial as that used for forming the reflective film 21 but unlike thecase of forming the reflective film 21 included in the L0 layer 20, itis unnecessary to consider the light transmittance of the material whena material is selected for forming the reflective film 31.

[0095] The optical recording medium 10 having the above-describedconfiguration can, for example, be fabricated in the following manner.

[0096] FIGS. 3 to 6 show the steps of a method for fabricating theoptical recording medium 10 according to this embodiment.

[0097] As shown in FIG. 3, the support substrate 11 having grooves 11 aand lands 11 b on the surface thereof is first fabricated by aninjection molding process using a stamper 40.

[0098] Then, as shown in FIG. 4, the reflective film 31, the fourthdielectric film 32, the L1 recording film 33 and the third dielectricfilm 34 are sequentially formed on the substantially entire surface ofthe support substrate 11 on which the grooves 11 a and the lands 11 bare formed by a gas phase growth process such as a sputtering process,thereby forming the L1 layer 30. The L1 recording film 33 is normally inan amorphous state immediately after formation by a sputtering processor the like.

[0099] Further, as shown in FIG. 5, an ultraviolet ray curable resin iscoated on the L1 layer 30 by a spin coating method to form a coatingfilm and the surface of the coating film is irradiated with anultraviolet ray via a stamper 41 while it is covered by the stamper 41,thereby forming the transparent intermediate layer 12 formed withgrooves 12 a and lands 12 b on the surface thereof.

[0100] Then, as shown in FIG. 6, the reflective film 21, the seconddielectric film 22, the L0 recording film 23 and the first dielectricfilm 24 are sequentially formed on substantially the entire surface ofthe transparent intermediate layer 12 on which the grooves 12 a and thelands 12 b are formed, by a gas phase growth process such as asputtering process, thereby forming the L0 layer 20. The L0 recordingfilm 23 is normally in an amorphous state immediately after formation bya sputtering process or the like.

[0101] An ultraviolet ray curable resin is further coated on the L0layer 20 by a spin coating method to form a coating film and the surfaceof the coating film is irradiated with an ultraviolet ray, therebyforming the light transmission layer 13.

[0102] This completes the fabrication of the optical recording medium10.

[0103] The optical recording medium 10 is then set on a rotatable tableof a laser illumination apparatus (not shown) and while the rotatabletable is being rotated, the optical recording medium 10 is continuouslyirradiated with a laser beam L having such a rectangular beamcross-section whose length is short in the direction along a track(circumferential direction of the optical recording medium 10) and whoselength is long in a direction perpendicular to the track (radialdirection of the optical recording medium 10) in such a manner that theposition irradiated with the laser beam is dislocated in a directionperpendicular to the track every rotation of the optical recordingmedium 10, thereby irradiating substantially the entire surfaces of theL0 recording film 23 and the L1 recording film 33 with the laser beam Lof rectangular cross-section.

[0104] As a result, the phase change material contained in the L0recording film 23 and the L1 recording film 33 is heated to atemperature equal to or higher than the crystallization temperaturethereof and then gradually cooled, whereby substantially the entiresurfaces of the L0 recording film 23 and the L1 recording film 33 arecrystallized. In this specification, this process is referred to as “aninitialization process”. In this state, no data are recorded in the L0recording film 23 and the L1 recording film 33 and the L0 recording film23 and the L1 recording film 33 are in an unrecorded state.

[0105] When the initialization process has been completed in thismanner, there is obtained an optical recording medium 10 having the L0recording film 23 and the L1 recording film 33 in which no data arerecorded.

[0106] When data are to be recorded in the thus constituted opticalrecording medium 10, the light incident plane 13 a of the lighttransmission layer 13 is irradiated with a laser beam L whose intensityis modulated and the focus of the laser beam L is adjusted onto the L0recording film 23 or the L1 recording film 33.

[0107] When a predetermined region of the L0 recording film 23 or the L1recording film 33 is heated by the irradiation with the laser beam L toa temperature equal to or higher than the melting point of the phasechange material and quickly cooled, the region assumes an amorphousstate. On the other hand, when a predetermined region of the L0recording film 23 or the L1 recording film 33 is heated by theirradiation with the laser beam L to a temperature equal to or higherthan the crystallization temperature of the phase change material andgradually cooled, the region assumes a crystallized state. A record markis formed by the region in the amorphous state of the L0 recording film23 or the L1 recording film 33. The length of the record mark and thelength of the blank region between the record mark and the neighboringrecord mark in the direction of the track constitute data recorded inthe L0 recording film 23 or the L1 recording film 33.

[0108] In this embodiment, the reflective film 21 included in the L0layer 20 contains Ag as a primary component and C as an additive and hasa high light transmittance. Therefore, when data are recorded in the L1recording film 33, the data can be recorded in the L1 recording film 33in a desired manner. On the other hand, when data are recorded in the L0recording film 23, since the reflective film 21 included in the L0 layer20 has a high thermal conductivity, heat generated by the irradiationwith the laser beam is not excessively stored in the L0 recording film23 included in the L0 layer 20 and it is therefore possible to recorddata in the L0 recording film 23 in a desired manner.

[0109] On the other hand, when data recorded in the optical recordingmedium 10 are to be reproduced, the light incident plane 13 a of thelight transmission layer 13 is irradiated with a laser beam L whoseintensity is modulated and the focus of the laser beam L is adjustedonto the L0 recording film 23 or the L1 recording film 33.

[0110] Since the reflection coefficients of the L0 recording film 23 orthe L1 recording film 33 are different between a region in an amorphousstate and a region in a crystallized state, it is possible to reproducedata recorded in the L0 recording film 23 or the L1 recording film 33 bydetecting the amount of light reflected from the L0 recording film 23 orthe L1 recording film 33.

[0111] In this embodiment, since the reflective film 21 included in theL0 layer 20 contains Ag as a primary component and C as an additive andhas a high light transmittance and a high thermal conductivity, it ispossible to reproduce data recorded in the L1 recording film 33.

[0112] According to this embodiment, since the reflective film 21included in the L0 layer 20 is formed of a material containing Ag as aprimary component and C as an additive, it is possible to improve thelight transmittance and the thermal conductivity of the reflective film21 included in the L0 layer 20 while preventing the reflective film 21from being corroded and it is therefore possible to improve therecording characteristics and reproducing characteristics of the L0recording film 23 and the L1 recording film 33 and improve the storagereliability of the optical recording medium 10.

[0113]FIG. 7 is a schematic cross-sectional view showing an opticalrecording medium which is another preferred embodiment of the presentinvention.

[0114] As shown in FIG. 7, an optical recording medium 50 according tothis embodiment is constituted as a write-once type optical recordingmedium and includes a disk-like support substrate 11, a transparentintermediate layer 12, a light transmission layer 13, an L0 layer 60formed between the transparent layer 12 and the light transmission layer13, and an L1 layer 70 formed between the support substrate 11 and thetransparent intermediate layer 12.

[0115] The L0 layer 60 and the L1 layer 70 are recording layers in whichdata are recorded and the optical recording medium 50 according to thisembodiment includes two recording layers.

[0116] The L0 layer 60 constitutes a recording layer close to a lightincident plane 13 a and is constituted by laminating a reflective film61, a second dielectric film 62, a first L0 recording film 63 a, asecond L0 recording film 63 b and a first dielectric film 64 from theside of the support substrate 11.

[0117] On the other hand, the L1 layer 70 constitutes a recording layerfar from the light incident plane 13 a and is constituted by laminatinga reflective film 71, a fourth dielectric film 72, a first L1 recordingfilm 73 a, a second L1 recording film 73 b and a third dielectric film74.

[0118] In the case where data are to be recorded in the L1 layer 70 anddata recorded in the L1 layer 70 are to be reproduced, a laser beam L isprojected via the L0 layer 60 located close to the light transmissionlayer 13.

[0119] The optical recording medium 50 according to this embodiment hasthe same configuration as that of the optical recording medium 10 shownin FIG. 1 except that the L0 layer 60 is provided instead of the L0layer 20 and the L1 layer 70 is provided instead of the L1 layer 30.

[0120] Further, the L0 layer 60 of the optical recording medium 50according to this embodiment has the same configuration as that of theL0 layer 20 of the optical recording medium 10 shown in FIG. 1 exceptthat the first L0 recording film 63 a containing an element selectedfrom the group consisting of Si, Ge, Mg, Al and Sn as a primarycomponent and the second L0 recording film 63 b containing Cu as aprimary component are provided instead of the L0 recording film 23containing a phase change material.

[0121] Therefore, the reflective film 61 included in the L0 layer 60contains Ag as a primary component and C as an additive.

[0122] Similarly, the L1 layer 70 of the optical recording medium 50according to this embodiment has the same configuration as that of theL1 layer 30 of the optical recording medium 10 shown in FIG. 1 exceptthat the first L1 recording film 73 a containing an element selectedfrom the group consisting of Si, Ge, Mg; Al and Sn as a primarycomponent and the second L1 recording film 73 b containing Cu as aprimary component are provided instead of the L1 recording film 33containing a phase change material.

[0123] Therefore, the reflective film 71 included in the L1 layer 70contains Ag as a primary component and C as an additive.

[0124] Cu contained in the second L0 recording film 63 b or the secondL1 recording film 73 b as a primary component quickly mixes with theelement contained in the first L0 recording film 63 a or the first L1recording film 73 a when irradiated with a laser beam L, therebyenabling data to be quickly recorded in the L0 layer 60 or the L1 layer70.

[0125] In order to improve the recording sensitivity of the first L0recording film 63 a or the first L1 recording film 73 a, one or moreelements selected from a group consisting of Mg, Al, Cu, Ag and Au maybe further added to the first L0 recording film 63 a or the first L1recording film 73 a.

[0126] In order to improve the storage reliability and the recordingsensitivity of the second L0 recording film 63 b or the second L1recording film 73 b, at least one element selected from the groupconsisting of Al, Si, Zn, Mg and Au may be further added to the secondL0 recording film 63 b or the second L1 recording film 73 b. The amountof the element (elements) added to the second L0 recording film 63 b orthe second L1 recording film 73 b is preferably equal to or more than 1atomic % and less than 50 atomic %.

[0127] It is preferable to form the first L0 recording film 63 a and thesecond L0 recording film 63 b so that the total thickness thereof is 2nm to 15 nm and it is preferable to form the first L1 recording film 73a and the second L1 recording film 73 b so that the total thicknessthereof is 2 nm to 40 nm.

[0128] When the total thickness of the first L0 recording film 63 a andthe second L0 recording film 63 b or the total thickness of the first L1recording film 73 a and the second L1 recording film 73 b is less than 2nm, the change in reflection coefficient between before and afterirradiation with the laser beam L is small so that a reproduced signalhaving high strength cannot be obtained.

[0129] On the other hand, when the total thickness of the first L0recording film 63 a and the second L0 recording film 63 b exceeds 15 nm,the light transmittance of the L0 layer 60 is lowered and the recordingcharacteristic and the reproducing characteristic of the L1 layer 70 aredegraded.

[0130] Further, when the total thickness of the first L1 recording film73 a and the second L1 recording film 73 b exceeds 40 nm, the mixingrate of the element contained in the first L1 recording film 73 b as aprimary component and the element contained in the second L1 recordingfilm 73 b is low and it becomes difficult to record data at high speed.

[0131] Further, it is preferable to define the ratio of the thickness ofthe first. L0 recording film 63 a to the thickness of the second L0recording film 63 b (thickness of the first L0 recording film 63a/thickness of the second L0 recording film 63 b) and the ratio of thethickness of the first L1 recording film 73 a to the thickness of thesecond L1 recording film 73 b (thickness of the first L1 recording film73 a/thickness of the second L1 recording film 73 b) to be from 0.2 to5.0.

[0132] The optical recording medium 50 having the above-describedconfiguration can, for example, be fabricated in the following manner.

[0133] As shown in FIG. 3, the support substrate 11 having grooves 11 aand lands 11 b on the surface thereof is first fabricated by aninjection molding process using a stamper 40.

[0134] Then, similarly to what is shown in FIG. 4, the reflective film71, the fourth dielectric film 72, the second L1 recording film 73 b,the first L1 recording film 73 a and the third dielectric film 74 aresequentially formed on substantially the entire surface of the supportsubstrate 11 on which the grooves 11 a and the lands 11 b are formed, bya gas phase growth process such as a sputtering process, thereby formingthe L1 layer 70.

[0135] Further, similarly to what is shown in FIG. 5, an ultraviolet raycurable resin is coated on the L1 layer 70 by a spin coating method toform a coating film and the surface of the coating film is irradiatedwith an ultraviolet ray via a stamper 41 while it is covered by thestamper 41, thereby forming the transparent intermediate layer 12 formedwith grooves 12 a and lands 12 b on the surface thereof

[0136] Then, similarly to what is shown in FIG. 6, the reflective film61, the second dielectric film 62, the second L0 recording film 63 b,the first L0 recording film 63 a and the first dielectric film 64 aresequentially formed on substantially the entire surface of thetransparent intermediate layer 12 on which the grooves 12 a and thelands 12 b are formed by a gas phase growth process such as a sputteringprocess, thereby forming the L0 layer 60.

[0137] An ultraviolet ray curable resin is further coated on the L0layer 60 by a spin coating method to form a coating film and the surfaceof the coating film is irradiated with an ultraviolet ray, therebyforming the light transmission layer 13.

[0138] This completes the fabrication of the optical recording medium50.

[0139] When data are to be recorded in the thus constituted opticalrecording medium 50, the light incident plane 13 a of the lighttransmission layer 13 is irradiated with a laser beam L whose intensityis modulated and the focus of the laser beam L is adjusted onto thefirst L0 recording film 63 a and the second L0 recording film 63 b oronto the first L1 recording film 73 a and the second L1 recording film73 b.

[0140] As a result, the element contained in the first L0 recording film63 a as a primary component and Cu contained in the second L0 recordingfilm 63 b as a primary component or the element contained in the firstL1 recording film 73 a as a primary component and Cu contained in thesecond L1 recording film 73 b as a primary component are mixed with eachother and as shown in FIG. 8, a record mark M is formed.

[0141] In this manner, record marks M are formed in the L0 layer 60 orthe L1 layer 70 and data are recorded therein.

[0142] In this embodiment, since the reflective film 61 included in theL0 layer 60 contains Ag as a primary component and C as an additive andhas a high light transmittance, even when data are recorded in the L1layer 70, it is possible to record data in the L1 layer 70 in a desiredmanner. On the other hand, when data are recorded in the L0 layer 60,since the reflective film 61 included in the L0 layer 60 has a highthermal conductivity, heat generated by the irradiation with the laserbeam is not excessively stored in the first L0 recording film 63 a andthe second L0 recording film 63 b included in the L0 layer 60 and it istherefore possible to record data in the L0 layer 60 in a desiredmanner.

[0143] On the other hand, when data recorded in the optical recordingmedium 50 are to be reproduced, the light incident plane 13 a of thelight transmission layer 13 is irradiated with a laser beam L whoseintensity is modulated and the focus of the laser beam L is adjustedonto the L0 layer 60 or the L1 layer 70.

[0144] Since the reflection coefficient of the record mark M formed bymixing the element contained in the first L0 recording film 63 a as aprimary component and Cu contained in the second L0 recording film 63 bas a primary component or the element contained in the first L1recording film 73 a as a primary component and Cu contained in thesecond L1 recording film 73 b as a primary component is greatlydifferent from that of the region surrounding the region where therecord mark M is formed, it is possible to obtain a high reproducedsignal (C/N ratio) by detecting the amount of light reflected from theL0 layer 60 or the L1 layer 70.

[0145] According to this embodiment, since the reflective film 61included in the L0 layer 60 is formed of a material containing Ag as aprimary component and C as an additive, it is possible to improve alight transmittance and a thermal conductivity of the reflective film 61included in the L0 layer 60 while preventing the reflective film 61 frombeing corroded and it is therefore possible to improve the recordingcharacteristics and reproducing characteristics of the L0 layer 60 andthe L1 layer 70 and improve the storage reliability of the opticalrecording medium 50.

WORKING EXAMPLES

[0146] Hereinafter, working examples will be set out in order to furtherclarify the advantages of the present invention.

Working Example 1

[0147] An optical recording medium sample was fabricated in thefollowing manner.

[0148] A disk-like polycarbonate substrate having a thickness of 1.1 mmand a diameter of 120 mm was first fabricated by an injection moldingprocess. No grooves or lands were formed on the surface of thepolycarbonate substrate.

[0149] Then, the polycarbonate substrate was set on a sputteringapparatus and a reflective film containing Ag as a primary component andC as an additive and having a thickness of 8 nm, a second dielectricfilm containing a mixture of ZnS and SiO₂ and having a thickness of 32nm, a second recording film containing Si as a primary component andhaving a thickness of 5 nm, a first recording film containing Cu as aprimary component and having a thickness of 5 nm and a first dielectricfilm containing the mixture of. ZnS and SiO₂ and having a thickness of30 nm were sequentially formed on the polycarbonate substrate using thesputtering process.

[0150] Further, the first dielectric film was coated using the spincoating method with a resin solution prepared by dissolving acrylicultraviolet curing resin in a solvent to form a coating layer and thecoating layer was irradiated with ultraviolet rays, thereby curing theacrylic ultraviolet curing resin to form a protective layer having athickness of 100 μm.

[0151] The mole ratio of ZnS to SiO₂ in the mixture of ZnS and SiO₂contained in the first dielectric layer and the second dielectric layerwas 80:20.

[0152] In this manner, seven optical recording medium samples werefabricated with varying amounts of C added to the reflective film.

[0153] Each of the thus fabricated optical recording medium samples wasirradiated with a laser beam having a wavelength of 405 nm via the lighttransmission layer and the amount of the laser beam coming out from thepolycarbonate substrate was detected, thereby measuring the lighttransmittance of the reflective film of the optical recording mediumsample.

[0154] The results of the measurements are shown in FIG. 9.

[0155] As shown in FIG. 9, it was found that in the case where theamount of C added to the reflective film was equal to or less than about2.5 atomic %, the light transmittance of the reflective film wassubstantially the same as that of a reflective film formed of Ag onlybut that in the case where the amount of C added to the reflective filmexceeded about 2.5 atomic %, the light transmittance of the reflectivefilm gradually decreased as the amount of C added to the reflective filmincreased and that in the case where the amount of C added to thereflective film exceeded about 5.0 atomic %, the light transmittance ofthe reflective film became lower than 40%.

Working Example 2

[0156] Samples were fabricated in the following manner.

[0157] A disk-like polycarbonate substrate having a thickness of 1.1 mmand a diameter of 120 mm was first fabricated by an injection moldingprocess. No grooves or lands were formed on the surface of thepolycarbonate substrate.

[0158] Then, the polycarbonate substrate was set on a sputteringapparatus and a reflective film containing Ag as a primary component andC as an additive and having a thickness of 100 nm was formed on thepolycarbonate substrate using the sputtering process.

[0159] In this manner, six samples were fabricated with varying amountsof C added to the reflective film.

[0160] The reflective film of each of the thus fabricated samples wasirradiated with a laser beam having a wavelength of 405 nm and theamount of the laser beam reflected by the reflective film was detected,thereby measuring the reflection coefficient Rco of the reflective filmof the sample.

[0161] Further, each of the samples was held at a temperature of 80° C.and relative humidity of 85% for 50 hours, thereby conducting a storagetest on each of the samples. Following this test, the reflective film ofeach of the samples was again irradiated with a laser beam having awavelength of 405 nm and the amount of the laser beam reflected by thereflective film was detected, thereby measuring the reflectioncoefficient Rc of the reflective film of the sample subjected to thestorage test.

[0162] Based on the thus measured reflection coefficients of thereflective film of each of the samples, the rate of reflectioncoefficient reduction was evaluated. The rate of reflection coefficientreduction was defined as (Rco−Rc)×100/Rco (%).

[0163] The results of evaluation are shown in FIG. 10.

[0164] As shown in FIG. 10, it was found that the reflection coefficientafter the storage test was lower than the reflection coefficient beforethe storage test in each of the samples. This can be considered to bebecause Ag contained in the reflective film of each of the samples wascorroded by the storage test.

[0165] Further, it was found that the rate of reflection coefficientreduction was smaller as the amount of C added to the reflective filmincreased and that in the case where the amount of C added to thereflective film was less than about 2.0 atomic %, the rate of reflectioncoefficient reduction markedly decreased with increasing amount of Cadded to the reflective film.

Working Example 3

[0166] Samples were fabricated in the following manner.

[0167] A disk-like polycarbonate substrate having a thickness of 1.1 mmand a diameter of 120 mm was first fabricated by an injection moldingprocess. No grooves or lands were formed on the surface of thepolycarbonate substrate.

[0168] Then, the polycarbonate substrate was set on a sputteringapparatus and a reflective film containing Ag as a primary component and2.5 atomic % of C as an additive and having a thickness of 100 nm wasformed on the polycarbonate substrate using the sputtering process,thereby fabricating a sample #1.

[0169] A similarly fabricated polycarbonate substrate was set on asputtering apparatus and a reflective film containing Ag as a primarycomponent and 5.0 atomic % of C as an additive and having a thickness of100 nm was formed on the polycarbonate substrate using the sputteringprocess, thereby fabricating a sample # 2.

[0170] A similarly fabricated polycarbonate substrate fabricatedsimilarly was set on a sputtering apparatus and a reflective filmcontaining Ag as a primary component and 5.0 atomic % of Sn as anadditive and having a thickness of 100 nm was formed on thepolycarbonate substrate using the sputtering process, therebyfabricating a comparative sample # 1.

[0171] A similarly fabricated polycarbonate substrate fabricatedsimilarly was set on a sputtering apparatus and a reflective filmcontaining Ag as a primary component and 10.0 atomic % of W as anadditive and having a thickness of 100 nm was formed on thepolycarbonate substrate using the sputtering process, therebyfabricating a comparative sample # 2.

[0172] The electrical conductivity of the reflective film of each of thesamples #1 and #2 and the comparative samples #1 and #2 was measured bya four probe method and the thermal conductivity of the reflective filmof each sample was calculated using the Wiedemann-Franz law.

[0173] The measurement and calculation results are shown in Table 1.TABLE 1 Thermal Conductivity (W/mK) Sample #1 163.0 Sample #2 91.1Comparative Sample #1 42.8 Comparative Sample #2 12.5

[0174] As shown in Table 1, it was found that the reflective filmcontaining Ag as a primary component and C as an additive had a muchhigher thermal conductivity than that of the reflective film containingAg as a primary component and Sn as an additive and that of thereflective film containing Ag as a primary component and W as anadditive.

[0175] From the Working Examples 1 to 3, it was found that it waspreferable to form a reflective film of a material containing Ag as aprimary component and 0.5 atomic % to 5.0 atomic % of C as an additiveand is more preferable to form a reflective film of a materialcontaining Ag as a primary component and 1.0 atomic % to 4.0 atomic % ofC as an additive in order to form a reflective film having a high lighttransmittance, a high thermal conductivity and an excellent storagereliability Further, it was found that it was most preferable to form areflective film of a material containing Ag as a primary component andabout 2.5 atomic % of C as an additive in order to form a reflectivefilm having a high light transmittance, a high thermal conductivity andan excellent storage reliability.

[0176] The present invention has thus been shown and described withreference to specific embodiments. However, it should be noted that thepresent invention is in no way limited to the details of the describedarrangements but changes and modifications may be made without departingfrom the scope of the appended claims.

[0177] For example, in the embodiment shown in FIGS. 1 and 2, althoughthe optical recording medium 10 includes the L0 recording film 23 andthe L1 recording film 33 containing a SbTe system material, it is notabsolutely necessary for the optical recording medium 10 to include theL0 recording film 23 and the L1 recording film 33 containing a SbTesystem material and the optical recording medium 10 may include an L0recording film and an L1 recording film containing another phase changematerial.

[0178] Further, in the embodiment shown in FIGS. 7 and 8, although thefirst L0 recording film 63 a and the second L0 recording film 63 b ofthe L0 layer 60 are formed in contact with each other it is notabsolutely necessary to form the first L0 recording film 63 a and thesecond L0 recording film 63 b of the L0 layer 60 in contact with eachother but it is sufficient for the second L0 recording film 63 b to beso located in the vicinity of the first L0 recording film 63 a as toenable formation of a mixed region including the primary componentelement of the first L0 recording film 63 a and the primary componentelement of the second L0 recording film 63 b when the region isirradiated with a laser beam. Further, one or more other films such as adielectric film may be interposed between the first L0 recording film 63a and the second L0 recording film 63 b.

[0179] Furthermore, in the embodiment shown in FIGS. 7 and 8, althoughthe first L1 recording film 73 a and the second L1 recording film 73 bof the L1 layer 70 are formed in contact with each other it is notabsolutely necessary to form the first L1 recording film 73 a and thesecond L1 recording film 73 b of the L1 layer 70 in contact with eachother but it is sufficient for the second L1 recording film 73 b to beso located in the vicinity of the first L1 recording film 73 a as toenable formation of a mixed region including the primary componentelement of the first L1 recording film 73 a and the primary componentelement of the second L1 recording film 73 b when the region isirradiated with a laser beam.

[0180] Further, one or more other films such as a dielectric film may beinterposed between the first L1 recording film 73 a and the second L1recording film 73 b.

[0181] Moreover, in the embodiment shown in FIGS. 7 and 8, although theL0 layer 60 includes the first L0 recording film 63 a and the second L0recording film 63 b, the L0 layer 60 may include one or more recordingfilms containing the same element as a primary component as thatcontained in the first L0 recording film 63 a as a primary component orone or more recording films containing the same element as a primarycomponent as that contained in the second L0 recording film 63 b as aprimary component, in addition to the first L0 recording film 63 a andthe second L0 recording film 63 b.

[0182] Further, in the embodiment shown in FIGS. 7 and 8, although theL1 layer 70 includes the first L1 recording film 73 a and the second L1recording film 73 b, the L1 layer 70 may include one or more recordingfilms containing the same element as a primary component as thatcontained in the first L1 recording film 73 a as a primary component orone or more recording films containing the same element as a primarycomponent as that contained in the second L1 recording film 73 b as aprimary component, in addition to the first L1 recording film 73 a andthe second L1 recording film 73 b.

[0183] Furthermore, in the embodiment shown in FIGS. 7 and 8, althougheach of the first L0 recording film 63 a and the first L1 recording film73 a contains an element selected from the group consisting of Si, Ge,Mg, Al and Sn as a primary component and each of the second L0 recordingfilm 63 b and the second recording film 73 b contains Cu as a primarycomponent, it is not absolutely necessary for each of the first L0recording film 63 a and the first L1 recording film 73 a to contain anelement selected from the group consisting of Si, Ge, Mg, Al and Sn as aprimary component and for each of the second L0 recording film 63 b andthe second recording film 73 b to contain Cu as a primary component.Each of the first L0 recording film 63 a and the first L1 recording film73 a may contain an element other than Si, Ge, Mg, Al and Sn as aprimary component and each of the second L0 recording film 63 b and thesecond recording film 73 b may contain an element other than Cu as aprimary component.

[0184] Moreover, in the embodiment shown in FIGS. 7 and 8, although thefirst L0 recording film 63 a is disposed on the side of the lighttransmission layer 13 and the second L0 recording film 63 b is disposedon the side of the support substrate 11, it is possible to dispose thefirst L0 recording film 63 a on the side of the support substrate 11 andthe second L0 recording film 63 b on the side of the light transmissionlayer 13.

[0185] Further, in the embodiment shown in FIGS. 7 and 8, although thefirst L1 recording film 73 a is disposed on the side of the lighttransmission layer 13 and the second L1 recording film 73 b is disposedon the side of the support substrate 11, it is possible to dispose thefirst L1 recording film 73 a on the side of the support substrate 11 andthe second L1 recording film 73 b on the side of the light transmissionlayer 13.

[0186] Furthermore, although the optical recording medium 10 includesthe L0 recording film 23 and the L1 recording film 33 in the embodimentshown in FIGS. 1 and 2 and the optical recording medium 50 includes theL0 layer 60 and the L1 layer 70 in the embodiment shown in FIGS. 7 and8, it is not absolutely necessary for the optical recording medium toinclude two recording layers but the optical recording medium mayinclude three or more recording layers.

[0187] Further, in the above described embodiment, although thereflective film 21 of the L0 layer 20 or the reflective film 61 of theL0 layer 60 is formed on the transparent intermediate layer 12, it ispossible to provide a protective layer having a thickness of 2 nm to 150nm and made of a material for forming the first dielectric film 24, 64between the transparent intermediate layer 12 and the reflective film 21or the reflective film 61 and physically space the reflective film 21 orthe reflective film 61 and the transparent intermediate layer 12,thereby preventing the transparent intermediate layer 12 from beingdamaged by heat when data are recorded in the L0 layer 20 or the L0layer 60.

[0188] Furthermore, in the above described embodiment, although thelight transmission layer 13 is formed on the surface of the firstdielectric film 24 of the L0 layer 20 or the first dielectric film 64 ofthe L0 layer 60, it is possible to provide a transparent heat radiationfilm having a thickness of 10 nm t0 200 nm and made of a material havinghigher thermal conductivity than that of the material forming the firstdielectric film 24, 64 between the first dielectric film 24 of the L0layer 20 or the first dielectric film 64 of the L0 layer 60 and thelight transmission layer 13 in order to improve heat radiationcharacteristics of the L0 layer 20 or the L0 layer 60 and it is furtherpossible to provide a dielectric film having a different refractiveindex from that of the transparent heat radiation film between thetransparent heat radiation film and the light transmission layer 13 inorder to increase light interference effect.

[0189] Moreover, although the optical recording medium 10 includes theL0 recording film 23 and the L1 recording film 33 each containing aphase change material in the embodiment shown in FIGS. 1 and 2 and theoptical recording medium 50 includes the L0 layer 60 including the firstL0 recording film 63 a and the second L0 recording film 63 b eachcontaining an inorganic element as a primary component and the L1 layer70 including the first L1 recording film 73 a and the second L1recording film 73 b each containing an inorganic element as a primarycomponent, the present invention is not limited to optical recordingmedia having such configurations and can be widely applied to an opticalrecording medium having a plurality of recording layers.

[0190] According to the present invention, it is possible to provide anoptical recording medium having a plurality of recording layers and inwhich recording characteristics and reproducing characteristic of therespective recording layers can be improved.

1. An optical recording medium comprising a substrate, a lighttransmission layer and a plurality of recording layers between thesubstrate and the light transmission layer and capable of recording datain the plurality of recording layers and reproducing data recorded inthe plurality of recording layers by projecting a laser beam via thelight transmission layer onto the plurality of recording layers, atleast one recording layer other than a farthest recording layer from thelight transmission layer including a reflective film containing Ag as aprimary component and C as an additive.
 2. An optical recording mediumin accordance with claim 1, wherein each of the plurality of recordinglayers contains a phase change material.
 3. An optical recording mediumin accordance with claim 1, wherein each of the plurality of recordinglayers includes a first recording film containing one kind of elementselected from the group consisting of Si, Ge, Sn, Mg, C, Al, Zn, In, Cuand Bi as a primary component and a second recording film disposed inthe vicinity of the first recording film and containing one kind ofelement from the group consisting of Cu, Al, Zn, Si and Ag and differentfrom the element contained in the first recording film as a primarycomponent and the element contained in the first recording film as aprimary component and the element contained in the second recording filmas a primary component are mixed when the first recording film and thesecond recording film is irradiated with a laser beam, thereby forming arecord mark.
 4. An optical recording medium in accordance with claim 2,wherein the reflective film included in the at least one recording layercontains 0.5 atomic % to 5.0 atomic % of C.
 5. An optical recordingmedium in accordance with claim 3, wherein the reflective film includedin the at least one recording layer contains 0.5 atomic % to 5.0 atomic% of C.
 6. An optical recording medium in accordance with claim 4,wherein the reflective film included in the at least one recording layercontains 1.0 atomic % to 4.0 atomic % of C.
 7. An optical recordingmedium in accordance with claim 5, wherein the reflective film includedin the at least one recording layer contains 1.0 atomic % to 4.0 atomic% of C.
 8. An optical recording medium in accordance with claim 6,wherein the reflective film included in the at least one recording layercontains about 2.5 atomic % of C.
 9. An optical recording medium inaccordance with claim 7, wherein the reflective film included in the atleast one recording layer contains about 2.5 atomic % of C.
 10. Anoptical recording medium in accordance with claim 8, wherein the lighttransmission layer has a thickness of 30 μm to 200 μm.
 11. An opticalrecording medium in accordance with claim 9, wherein the lighttransmission layer has a thickness of 30 μm to 200 μm.